The lethargy distributions \(J_{\hat{\mathbf {z}}}^{+}f(u)\)  at the
origin of the flight path (\(z={0}{}\) ) for both cases are
shown in Fig. REF , and the values of the integral
partial current as a function of the flight length \(z\)  are shown in
Fig. REF . Both figures also show the relative difference
between the resampled sources and the OpenMC SSF. Also, a
summary with the integral values for different energy and angular
ranges is reported in Table REF . The large
relative differences shown for the first case can be attributed to the
high value of the second derivative in those lethargy regions, as
discussed in [1]}, [2]}. The
importance factor used for the lethargy variable in the second case
reduces the broadening of the lethargy distribution, showing a good
agreement with the original SSF. This can also be observed from the
integral values shown in Table REF , since the
relative difference is larger in the first case than in the second one
for all the chosen ranges.
<FIGURE><TABLE>